In the fields of a solar cell, a flat panel display (FPD), a light emitting device such as a light emitting diode (LED), a touch panel, and the like, a metal oxide film is formed on a substrate. As a method of forming a metal oxide film on a substrate by using a non-vacuum film formation technique, a metal oxide film formation method in which a solution is opened to air, such as a spray method, a sol-gel method, or a mist chemical vapor deposition (CVD) method, is conventionally employed.
In each of those film formation methods, when a metal oxide film which is an object being formed is formed, one kind of raw-material solution is used. While it is possible to perform blending for a raw-material solution beforehand, one kind of blended raw-material solution is supposed to be used in forming a metal oxide film.
As such, in a metal oxide film formation method using a conventional non-vacuum film formation technique, because of the use of one kind of blended raw-material solution in forming a metal oxide film, reaction energy required to form a metal oxide film with a raw-material solution is uniquely determined.
As a method which lowers reaction energy in forming a metal oxide film with a raw-material solution to increase a speed of forming a metal oxide film, there are two conventional methods as follows.
First, as a first method in which a process is performed in a direct manner, a method in which a reaction aiding agent is previously mixed into a raw-material solution, to lower the above-described reaction energy, is conceivable. As an example of the first method, a metal oxide film formation method disclosed in Patent Document 1 is cited, for example.
FIG. 7 is an explanatory view showing an overall configuration of a conventional film forming apparatus 200 using the first method, which is disclosed in Patent Document 1.
As shown in FIG. 7, a raw-material solution 18 (a solution including zinc) and a reaction aiding solution 24 (ammonia water) in the same solution vessel 15 are fed, and a mixed solution 34 of the raw-material solution 18 and the reaction aiding solution 24 is turned into a mist by a mist former 16, so that a mixed mist M4 which is the mixed solution 34 turned into a mist is obtained.
Then, the mixed mist M4 passes through a path L11 and is fed to a back surface (upper surface) of a P-type silicon substrate 4 in a reactor vessel 11, so that a back-side passivation film 5 (zinc oxide thin film) is formed on a back surface of the P-type silicon substrate 4.
According to the film formation method disclosed in Patent Document 1, in forming a metal oxide film (zinc oxide thin film) using a mist method, the raw-material solution 18 and the reaction aiding solution 24 including a reaction aiding agent (ammonia water) are mixed with each other in the same atomizer (the solution vessel 15, the mist former 16), so that the mixed mist M4 is obtained. Then, the mixed mist M4 is fed to a back surface (one main surface) of the P-type silicon substrate 4 in the reactor vessel 11, in an attempt to lower the above-described reaction energy.
On the other hand, a second method in which a process is performed in an indirect manner is a method in which a reaction aiding gas is additionally fed to the above-described reactor vessel to adjust an atmosphere at a time of formation of a metal oxide film, so that lowering of the above-described reaction energy at a time of formation of a metal oxide film can be achieved by a raw-material solution turned into a mist in the above-described reactor vessel, and a high-quality metal oxide film is attained.